Method of initiating polymerization electrolytically

ABSTRACT

POLYMERIZATION IS INITIATED ELECTROLYTICALLY BY PASSING A CURRENT THROUGH AN ELECTROCHEMICAL SYSTEM COMPRISED OF A METALLIC ANODE, A CATHODE, THE ANODE AND CATHODE BEING IMMERSED IN AN ELECTROLYTIC SOLUTION CONTAINING AN ELECTROLYTE AND POLYMERIZABLE MONOMERS, THE ELECTROLYTIC SOLUTION BEING INERT TOWARD SAID ANODE ON MERE CONTACT THEREWITH, BUT SOLUBILIZING THE ANODE WHEN AN ELECTRIC CURRENT IS PASSED. THE METAL IONS FORMED ON SOLUBILIZATION OF THE ANODE UPON ELECTROLYSIS ARE OXIDIZED WITH A COMPOUND WHICH FORMS FREE RADICALS, INITIATING POLYMERIZATION.

United States Patent 3,573,180 METHOD OF INITIATING POLYMERIZATION ELECTROLYTICALLY Harvey A. Hodes, James F. Sobieski, and Michael C. Zerner, Eatontown, N.J., assignors to the United States of America as represented by the Secretary of the Army No Drawing. Filed Nov. 3, 1967, Ser. No. 681,062 Int. Cl. B011: 3/00; C07b 29/ 06' US. Cl. 204-72 12 Claims ABSTRACT OF THE DISCLOSURE Polymerization is initiated electrolytically by passing a current through an electrochemical system comprised of a metallic anode, a cathode, the anode and cathode being immersed in an electrolytic solution containing an electrolyte and polymerizable monomers, the electrolytic solution being inert toward said anode on mere contact therewith, but solubilizing the anode when an electric current is passed. The metal ions formed on solubilization of the anode upon electrolysis are oxidized with a compound which forms free radicals, initiating polymerization.

The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to us of any royalty thereon.

BACKGROUND OF THE INVENTION This invention relates in general to a method of initiating polymerization electrolytically and, in particular, to such a method involving dissolution of a metallic anode to form metal ions which in turn react with a compound to form free radicals which initiate polymerization.

It is known that polymerization can be initiated electrically. Such polymerizations however have not been entirely satisfactory in that they lacked current sensitivity; that is, one electron could at best only form one free radical.

The object of this invention is to provide a method of initiating polymerization electrolytically which increases current sensitivity.

SUMMARY OF THE INVENTION According to this invention, a current is passed through an electrochemical system including a solution of electrolyte containing polymerizable monomer. Metal ions formed by solubilization of some of the anode material are then reacted with a compound, the reaction forming free radicals which initiate polymerization.

The electrochemical system through which the current is passed is comprised of a metallic anode that is readily oxidized or solubilized only upon the passage of current, and is not affected by mere contact with the electrolyte. Suitable metallic anodes include such metals as iron, copper, silver or chromium. Spaced from the anode is a cathode that is inert with repsect to the electrochemical system. Suitable cathodes include iron, platinum, copper or graphite.

The solution of electrolyte with which the anode and cathode are in contact includes an ethylenic unsaturated polymerizable monomer which may be, for example, acrylamide, N-N methylene bisacrylamide, metal acrylates, N-vinyl-Z-pyrrolidone or combinations thereof. The polymerizable monomer amounts to 3 to 50 percent by weight of the solution. The solution also includes certain salts that not only carry the current but increase the efiiciency of solution of the anode material when a potential is applied. Some examples are alkali chlorides which increase the efficiency of solution of an iron anode, and nitrates which are effective in aiding the solution of a cop ice per anode. Slightly acidic conditions generally increase the oxidation and solution of most anodes for a given current, but too strong an acid concentration may cause solution in the absence of current. The optimum pH depends on the particular system of interest, that is, the particular electrodes used, and the particular salts included in the solution.

Once metal ions have been introduced into the media, polymerization can be initiated by several techniques. These can be classified in three general groups: (1) oxidation of the metal ion to a higher valence state; (2) reduction to a lower valence state, with subsequent oxidation of the metal ions; and (3) oxidation-reduction reactions catalyzed by metal ions. All techniques must, of course, lead to the formation of free radicals capable of initiating polymerization of the included monomers. The three techniques are very often inter-related; indeed, the third technique is often present, or can be made a factor of the other two, and will not be discussed separately.

If metal ions from the anode go into solution in a low valence state, and the metal has a higher available valence state, the first technique is used. Examples are found with silver, tin, cobalt, etc., electrodes; i.e.,

wherein e is an electron.

The metal ions are then oxidized with a per compound, as peroxides, hydroperoxides, perborates, persulfates, etc., which, upon reaction form free radicals which initiate polymerization. For example,

The dot after S04. indicates the formation of free radicals. The produce sulfate radicals (50 7.) are capable of causing vinyl polymerization. It should be noted that very often a further reaction occurs linking the first technique with the third; i.e.,

The hydroxyl radicals ('OH) can also initiate polymerization, and the regenerated Ag+ reacts again with excess persulfate ion. In this respect, the Ag+ ion acts as an active catalyst according to the reaction.

Ag+ S2OB 2+ H20 $04 11* +'O r If the anode metal goes into solution at its higher state of oxidation, and a lower state is available which can react with a per-compound to form free radicals, the second processing technique is used. Iron, copper and chromium anodes are examples in which the more prevalent oxida tion state is the higher; i.e.,

The metal ion is then reduced, preferably with a reducing agent that will not directly react with the percompound used later; i.e.,

Fe+ +Reducing Agent Fe+ (Reducing Agent)+ The resulting reduced ion is then oxidized with a percompound as with the first technique; i.e.,

Again, if the reducing agent is in excess the Fe+ appears as an active catalyst for t +3 H202 +Redueing Agent OH- (Reducing Agent)+ OH This invention has many applications, and is especially appropriate in photoelectrolytic imaging devices. In such devices, the amount of current which flows through the electrolytic media is made proportional to the incident light via a photoconductive layer. Since more of the anode will dissolve with increased current passage, and since the amount of polymerization is made proportional to the amount of metal ion present by the described processing techniques, the amount of polymer formed is proportional to the amount of light, and can be made to leave a polymer image.

Simply devised photoelectric imaging devices using these described ideas quickly and reliably form high quality images, and the films are stable for long periods of time in the absence of current. Such devices are disclosed and claimed in the US. patent applications of Hodes and Zerner, Ser. No. 670,815 and Ser. No. 670,816 filed Sept. 26, 1967, and assigned to the same assignee, both now abandoned.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1 To a solution containing 0.1 N KNO in water is added 20 percent by weight of acrylamide and 4 percent by weight of N-N' ethylene bisacrylamide. A pair of electrodes separated by 1 centimeter and of 1 square centimeter in area are immersed in approximately 5 ml. of the monomer containing solution. The anode is silver and the cathode is platinum. A potential of 45 volts is applied across the electrodes and current past for 0.1 second. The electrodes are then removed. One drop of a percent by Weight solution of potassium persulfate is sufficient to initiate polymerization in about -60 seconds.

free radicals initiate polymerization. The following reaction is noted which regenerates Ag ions to further react with the added persulfate This reaction, which also produces free radicals, is apparently slow. If, after electrolysis, the pH of the solution is adjusted to pH 11 by addition of ammonium hydroxide, and one drop of 37 percent by weight of formaldehyde is added after the persulfate is added, the formaldehyde serves to reduce Ag+ to Ag+ making the reaction faster and more current sensitive. The pH is important, as reduction from Ag+ to black colloidal silver is possible in neutral or acid media, or more basic media. The reaction may well be To an aqueous solution of 0.1 normal KNO is added 20 percent by weight of calcium acrylate and 1 percent by weight of N-N'methylene bisacrylamide. The pH is adjusted with HCl to pH 56, and the solution saturated with diphenylamine. Two electrodes are immersed in 5 m1. of the above solution. For this example, the electrodes are kept 1 cm. apart and are 1 cm. in area. The anode is copper. The particular cathode material is not important, but for this example copper is used. A potential of volts is applied across the electrodes, and current is passed for 0.1 second. The electrodes are removed and one drop of 3 percent hydrogen peroxide is added. Polymerization ensues within 20 seconds. The polymer formed is white, due to the calcium acrylate. The reaction is 2Cu+ +2 diphenylamine-a ZCu+ +tetraphenylhydrazine+2H+ The addition of hydrogen peroxide then reacts with Cu+ to yield OH which initiates polymerization In the presence of excess diphenylamine, the regenerated Cu+ is again reduced to Cu+ where it again reacts with the hydrogen peroxide, etc.

+2 211202 2 diphenylamine tetraphenylhydrazine+2H2O +2H0' The reaction thus appears as one directly between hydrogen peroxide and diphenylamine catalyzed by Cu+ ions.

Example 3 To an aqueous solution of 0.1 normal KCl is added 20 percent acrylamide and 10 percent N-N' trimethylene bisacrylamide, both ingredients by weight. The pH is adjusted to pH 4-5 by addition of I-ICl. The solution is saturated with diphenylcarbazide. Two steel electrodes are immersed in this solution. For this example, the electrodes, 1 cm. apart and each 1 square centimeter in area, are immersed in 5 ml. of the above solution and a potential of 45 volts applied for 0.1 second. Addition of one drop of 1 percent hydrogen peroxide causes polymerization within 5 seconds at pH 4, and within 20 seconds at pH 5. The reactions involved are similar to those given in Example 2, but with Fe replacing Cu+ Fe replacing Cu+, and diphenylcarbazide and its oxidized product replacing diphenylamine and its oxidized products.

The foregoing description is to be considered merely as illustrative of the invention and not in limitation thereof.

What is claimed is:

1. A method of initiating polymerization electrolytically in an electrochemical system, said method including the steps of:

(A) preparing an aqueous solution containing polymerizable monomer and electrolyte;

(B) immersing a metallic anode and a cathode into said solution, wherein said metallic anode is comprised of a metal that dissolves principally in the lower of two readily available valence states and is readily oxidized only upon the passage of a current therethrough, and wherein said cathode is spaced from said anode and wherein said cathode is inert with respect to said electrochemical system;

(C) passing a current through said electrochemical system; and

(D) adding a per-compound to the electrochemical system whereby the metal ions formed are oxidized, said oxidization forming free radicals which initiate polymerization.

2. A method according to claim 1 wherein the metallic anode is comprised of a metal selected from the group consisting of silver and cobalt.

3. A method according to claim 1 wherein the salt of the electrolyte is selected from the group consisting of water soluble chlorides and nitrates.

4. A method according to claim 1 wherein the ethylenic unsaturated polymerizable monomer is selected from the group consisting of acrylamide, N-N' methylene bisacrylamide, metal acrylates and N-vinyl-Z-pyrrolidone.

5. A method according to claim 1 wherein the percompound is selected from the group consisting of peroxides, hydroperoxide, perborates, and persulfates.

6. A method according to claim 1 wherein the metallic ions formed by passing a current through the electrochemical systems are again reduced to a lower oxidization state after having been oxidized by free radical forming compounds; the reduced metal ions again being available for reaction and formation of free radicals.

7. A method according to claim 6 wherein the metallic anode is silver, the electrolyte is 0.1 N potassium nitrate, the polymerizable monomer is a mixture of acrylamide and N-N' ethylene bisacrylamide, and the per-compound is potassium persulfate,

8. A method of initiating polymerization electrolytically in an electrochemical system, said method including the steps of:

(A) preparing an aqueous solution containing polymerizable monomer, electrolyte, and reducing agent;

(B) immersing a metallic anode and a cathode into said solution, wherein said metallic anode is comprised of a metal that dissolves principally in the higher of two readily available valence states and is readily oxidized only upon the passage of a current therethrough, and wherein said cathode is spaced from said anode and wherein said cathode is inert with respect to said electrochemical system;

(C) passing a current through said electrochemical system;

(D) reducing the metallic ions formed to a lower oxidation state; and

(E) adding a per-compound to the electrochemical system whereby the reduced ions are oxidized, said oxidation forming free radicals which initiate polym erization.

9. A method according to claim 8 wherein the metal ions are again reduced by an excess of reducing agent to a lower oxidation state after having been oxidized by the per-compound; the reduced metal ions again being available for reaction and formation of free radicals.

10. A method according to claim 9 wherein the metallic anode is copper, the electrolyte is 0.1 N potassium nitrate,

the polymerizable monomer is a mixture of calcium acrylate and N-N' methylene bisacrylamide, the reducing agent is diphenylamine, and the per-compound is hydrogen peroxide.

11. A method according to claim 9 wherein the metallic anode is steel, the electrolyte is 0.1 N potassium chloride, the polymerizable monomer is a mixture of acrylamide and N-N trimethylene bisacrylamide, the reducing agent is diphenylcarbazide, and the per-compound is hydrogen peroxide.

12. A method according to claim 8 wherein the metallic anode is comprised of a metal selected from the group consisting of copper, iron, and chromium.

References Cited UNITED STATES PATENTS 3,434,946 3/1969 Ehrig et al. 204-72 3,375,237 3/1968 Baizer 260-88.7 3,140,276 7/1964 Forster 260-88.7 3,464,960 9/1969 Sobieski et al. 20472 3,402,112 9/1968 Brubaker -et al. 20474 2,726,204 12/1955 Park et al. 20472 OTHER REFERENCES Organic Chemistry, by Ray Q. Brewster, 3rd edition, 1961, pp. 779-780.

TA-HSUNG TUNG, Primary Examiner R. L. ANDREWS, Assistant Examiner 

